1. Field of the Invention
The present invention relates in general to a resistor element, and more particularly to a resistor element utilizing temperature dependence of an electrical resistance of an electrically resistive body, such as a temperature sensing element for measuring the temperature of a gaseous fluid, for example.
2. Discussion of the Prior Art
A resistor element such as a temperature sensing element which utilizes temperature dependence of an electrical resistance is known. FIG. 1 shows an example of this type of resistor element, which includes a ceramic tube 2 formed of alumina and having an outside diameter of about 0.5 mm, and electrical conductors or leads 4 made of platinum and having a diameter of about 0.2 mm. The leads 4 are secured to opposite end portions of the ceramic tube 2 by glass fillers 6. The resistor element further includes an electrically resistive body in the form of an extremely thin platinum wire 8 having a diameter of about 20 .about. 40 .mu.m, which is wound up about 100 turns on an outer circumferential surface of the ceramic tube 2. The opposite ends of the platinum wire 8 are coiled around the leads 4, and the leads and wire 4, 8 are welded together for electrical connection therebetween. The thus constructed resistor element is entirely covered by a protective coating made of a glass material.
There is also known a thin-film type of a resistor element as shown in FIG. 2. This resistor element has a platinum film 10 which is suitably patterned so as to have a desired resistance value, and which is formed on the outer surface of the ceramic tube 2, in place of the platinum wire 8 described above. This platinum film 10 is electrically connected to the leads 4, 4, by platinum connectors 12 which are provided on the opposite end faces of the ceramic tube 2 and the corresponding end faces of the glass fillers 6. The platinum connectors 12 are formed by baking a platinum paste (electrically conductive paste including platinum as a major component).
In practical use of the resistor element constructed as described above, the leads 4 are secured by welding to metallic rods, for example, so that the resistor element is placed in an intended position in a device. Referring to FIG. 3, for example, the resistor element 18 is disposed within a gas passage 16 which is defined by a pipe 14 such as an iron pipe, so as to measure the temperature of a gaseous fluid which flows through the passage 16. In this case, the leads 4 of the resistor element 18 are secured by welding at their opposite end portions to corresponding metallic rods 22, 22, such as stainless steel rods having a diameter of 2 mm, which are inserted into the pipe 14 through respective electrically insulating ceramic masses 20, 20 that fill holes formed through a wall of the pipe 14. Thus, the resistor element 18 is held in position within the pipe 14, with the metallic rods 22 being connected to an external device 24 such as a temperature indicator.
However, it is rather difficult for the known resistor element as described above to measure the temperature or other parameters of a measurement fluid with sufficiently high accuracy, when the environment for the measurement is rapidly changed. More specifically described by reference to FIG. 3, even if the temperature of a fluid to be measured, such as air, is rapidly changed, the temperature of the metallic rods 22 exposed to the measurement fluid remains unchanged, since the rods 22 have a relatively large heat capacity and is therefore unlikely to be highly responsive to the rapid change of the temperature of the fluid. As a result, the rate of temperature change of the resistor element 18 with the varying fluid temperature is low, due to heat transfer from the electrically resistive body 8, 10 to the metallic rods 22, or vice versa, through the leads 4. Namely, the temperature change of the resistor element 18 cannot follow the rapid change of the temperature of the fluid to be measured. Thus, the known resistor element suffers from reduced detecting accuracy upon a rapid change of the temperature of the measurement fluid.
To improve the operating response of the resistor element when the temperature of the measurement fluid is rapidly changed, the inventors of the present invention tried using a material having a lower thermal conductivity than platinum, for the leads of the resistor element which had conventionally been formed of platinum. In the resistor element of FIG. 2, for example, the leads 4 in the form of platinum wires were replaced by stainless steel wires having the same diameter as the platinum wires, to prepare an intended resistor element having an improved operating response. The stainless steel wires of the thus prepared resistor element were secured by spot welding (a type of resistance welding) to the metallic rods 22 as shown in FIG. 3, and the temperature of the fluid in the pipe 14 was actually measured. The inventors found some abnormal values in the measurements obtained by this resistor element.
Further study and analysis by the present inventors revealed that the abnormal values in the result of the measurement were caused by poor electrical conduction between the leads (stainless steel wires) and the electrically resistive body. When the leads (stainless steel wires) 4 of the resistor element of FIG. 2 and the metallic rods 22 are secured to each other by resistance welding, a considerable amount of flexural, bending or tensile force is applied to the leads 4 from the resistance welding electrodes. Since the leads 4 cannot endure such flexural or tensile force, the leads 4 tend to be more or less pulled out of the glass fillers 6, causing unfavorable clearances and cracks between the leads 4 and the glass fillers 6, which result in poor contact between the leads 4 and the platinum connectors 12 (i.e., poor electrical conduction between the leads 4 and the platinum film 10).
The above-described problem is often encountered in the case where the leads are formed of stainless steel or other material having a relatively low thermal conductivity. When the conventional platinum wires are used as the leads, on the other hand, the leads tend to be cut off rather than being pulled out of the glass fillers, when the excessive force is applied to the leads.